Drilling fluid circulation process and system



May 6,1958 R. A. 5080 2,833,517 I DRILLING FLUID CIRCULATION PROCESS AND SYSTEM Filed June 14, 1954 SHALE JHALE L. 1154px @l/A/(ER if INVENTOR.

A T TOR/YE Patented May 6, 1958 DRILLING FLUID CIRCULATION PROCESS AND SYSTEM Roy AQBobo, Houston, Tex, assignor toPhillips Petroleum Company, a corporation of Delaware Application June 14, 1954, Serial No. 436,446 18 Claims. or. zss-1.s

This invention relates to novel drilling fluid circulation processes and systems. In one aspect it relates to such processes and systems in which a liquid drilling mud isbeing circulated during rotary drilling, and the liquid is being lightened by the introduction of gases in Foran et al. 2,082,329,1une l, 1937. In another aspect it has particular relation to the introduction of the gas into the well at an intermediate point during certain pressure conditions.

In drilling wells for oil and gas, it is sometimes desirable to lighten the column of drilling fluid, to prevent lost circulation, to increase the drilling rate, and/or to allow small amounts of oil and gas from the formation being drilled to enter the drilling fluid so that it may be detected. This lightening of the drilling fluid is done by injecting a gas (generally air but sometimes hydrocarbon gas, or gases resulting from combustion) into the liquid used as the drilling fluid before it is pumped into the well. One difliculty encountered in this type of operation occurs when initiating gas flow, or when resuming circulation, as, for example, after making a roundtrip to change the bit. When the fluid containing compressed gas first leaves the lower end of the drill string, the gas rises and expands in the annular space. Since this space is full of drilling fluid, the gas causes a violent discharge of this liquid at the surface of the ground, resulting in a loss of liquid before the flow becomes stabilized. A second difliculty encountered in instances of lost circulation, where large quantities of the liquid drilling fluid are disappearing into permeable formation, is that when the gas emerges from the lower end of the drill string, it occupies a very small volume and may also be lost to-the formation along with the liquid drilling fluid, thus preventing any substantial weight reduction, or decrease of hydrostatic head, of the drilling liquid in the hole, and thus failing to stop the loss of drilling fluid.

The present invention avoids these difliculties of the prior art by providing a by-pass valve between the interior and exterior of the drill string atan intermediate point in the drill string operated by a motor responsive to the hydrostatic head of the drilling fluid in the well outside the drill string at the elevation of the valve as compared to a predetermined fluid pressure. This bypass valve reduces the pressure necessary to pump the gaslightened drilling liquid down through the drill string,

through the by-pass valve, and back through the annulus between drill string and the wall of the well bore to the surface, thus reducing the hydrostatic head at both the elevation of the valve and at the well bit, and making it easier to pump the gas lightened liquid all the way'down to the bit.- This isof a special value when relatively high gas-liquid ratios are used. In instances of lost circulation where the gas would also be lost if pumped out the lower end of the drill string initially, this allows the upper portion of the drilling liquid in the well bore above the valve to be lightened first, thus reducing the hydrostatic head; ofliquid in the well bore at the bit, so that the gas mayhth en occupy-a larger volume as it emerges from the I bottom of the drill string at the bit after the valve closes automatically and will not be lost to the formation along with the drilling liquid, but will then lighten the entire column of drilling liquid, thus reducing the excessive hy-.

drostatic pressure which has been causing the lost circulation, and thus ending the loss of drilling fluid through the formation, or at least substantially reducing said loss. This invention will also reduce or prevent the inconven-' ience and expense mentioned above caused by the gas producing a violent discharge of liquid at the surface of the ground before the flow becomes stabilized. It also reduces the size and pressure of the gas compressor necessary to pump gas out the bit. A

One object of the present invention is to provide a novel drilling fluid circulation process and system.

Another object is to reduce the pressure necessary to establish the circulation of gas along with the liquid drilling mud in order to lighten the same.

Another object is to reduce, or prevent lost circulation while drilling a well with drilling fluid by the rotary drilling process.

Another object is to increase'the rate of drilling.

Another object is to permit drilling with a relatively I high gas-mud ratio.

Further objects are to provide simple and rugged foolproof automatic equipment and systems for carrying out the above objects.

Numerous other objects and advantages will be apparent to those skilled in the art upon reading the accompanying drawings, specification and claims.

In the drawing, Figure 1 is a plan view, with parts broken away, of a rotary well drilling rigembodying the present invention.

Figure 2 is an elevational cross-sectional view taken through the well bore of a portion of the embodiment shown in Figure 1.

Figure 3 is a cross-sectional elevational enlarged view of the by-pass valve employed in the portion of the embodiment of the invention shown in Figure 2.

Figure 4 is an elevational cross-sectional view taken through the well bore and showing a drill string having a plurality of the by-pass valves of the present invention.

In Figure 1, a well generally designated as 4 is being drilled by the rotary method. A conventional derrick 6 is employed, employing a conventional rotary rig. This rotary rig may comprise any motor or prime mover 7 linked by any type of power transmission means 8 to rotary table 9 or other means for rotating the drill stem 11 in the well. While not absolutely necessary, it is conventional to have a well head 10 attached to a well casing or surface pipe around the drill stem 11 as it enters the well, said well head being provided with a lateral discharge pipe 12. Drill stem 11 (Figure 2) is attached by a conventional swivel connection (not shown) to a mud supply pipe 13. While the usual process employs circulation down the drill stem, many drilling operations arecarried out with reverse circulation, that is,

the drilling mud is pumped through pipe 12 through the well head (which may have a stufiing box or other packing around the drill stem) and emerges through drill stem 11 and pipe 13. As such is also conventional, it is deemed necessary to show only direct circulation and omit illustrating any reverse circulation. culation drilling mud 14 is taken from a part of tank 16 through intake pipe 17 by pump 18 and is forced through pipe 13 down drill stem 11, out through holes 46 in the drill bit 47 (Figure 2) and up the'well around drill stem 11 to emerge through pipe 12 and be returned through channel 19 to the tank 16.

Obviously the return conduits 12 and 19 need not be pipes but can be merely ditches in the ground, pipes being shown, however. 1

In direct cir- It. is customary to remove large cuttings by means of a vibrating screen 21 driven by motor 22, large cuttings going to a shale heap 23 and the fluid mud passing through pipes 19 into tank 16 where smaller cuttings may settle out to some degree. However, screen 21 can be removed and motor 22 stopped, in which case mud from 12 passes through 19 into tank 16 without removal of cuttings, the cuttings settling out in the tank, which may be aided by encountering baffles such as 24, accumulated settled-out cuttings then being shoveled out of tank 16 from time to time. Obviously the use of shale shaker 21 is preferable to shoveling out tank 16 due to the cost of labor involved, and the unpleasant nature of shoveling in the mud pit.

The derrick 6 has been shown by conventional symbols, the top of legs 26 being shown sawed off to'simplify the drawing. I I

The well drilling mud employed in tank 16 may be any conventional type of mud.

When employing a water base drilling mud, fresh water is added through pipe 27 provided, if necessary,

with pump 28 and/or valve 29. With an oil base mud, oil is added through pipe 27. Pipe 27 is preferably provided with a hopper 31communicating with the interior thereof'in order to add powdered or solid additives of various natures to the drilling mud. While these additives could be poured directly into tank 16, much better mixing occurs by adding them through hopper 31 into a stream of water flowing in pipe 27. If it is not desired to add more water, mud from tank 16 can be recycled through 31 to pick up the solids. These solids can include fiberous materials, or strips of paper, or the like (not shown) added to reduce loss of liquid to permeable formations when such loss is occurring.

Suitable bafllles 42 may be employed in tank 16 to direct the fluids flowing thereinto, and suchbafiies are generally movable as desired. Often the drilling mud in tank 16 is stirred up by jetting action of water. and/ or steam through a fire hose 43 having a nozzle 44.

Everything described with relation to Figure 1 up to this point is conventional in drilling wells by the rotary well drilling process, employing a liquid drilling fluid, sometimes called rotary well drilling mud because of the use of powdered additives added at hopper 31 and/ or the mud or clay coming from the formations drilled that becomes dissolved or dispersed in the drilling liquid making it a mud. In this application the terms liquid or fluid include muds. The novel portions will now be set forth using reference numerals above 50 to dis- 1.

tinguish the same from the conventional features having numerals below 50. However, novel combinations are formed between the novel portions and the conventional features of this invention.

In the above conventional rotary well drilling system comprising in combination a rotary well drill rig 6 including a first drilling liquid pump 18 having a first intake conduit 17 and a first discharge conduit 13 and a well drilling string 11 connected to said first discharge conduit 13 to receive liquid drilling fluid circulated by said first liquid pump there has been added a source of gas (in Figure 1, the source of gas is the atmosphere, but it could be any source of gas (not shown) such as a well producing hydrocarbon gas, or an engine or furnace producing combustion gases, or tanks containing either, or both of the same, along with air if desired), a gas compressor 51 having a second intake conduit 52 connected to said source of gas and a second discharge conduit 53 connected to said first discharge conduit 13 whereby compressed gas is combined with said liquid drilling fluid.

While discharge conduit 13 is shown connected to the top of drill stem 11 in the preferred form of the invention, it should be realized that drilling is possible with a reversed circulation, in which case conduit 13 is conpress said gas and further raise the pressure on said drilling fluid. When a second liquid pump 54 is employed it has at least two advantages. First, it enables the use of a smaller, lower pressure, and less expensive compressor 51 by doing some of the work of compressing the gas in pipe 53 which has been partially compressed by pump 51. Second, it helps make the mixture of gas and liquid being pumped in conduit 13 more homogeneous and helps disperse the gas more uniformly in greater numbers of smaller diameter bubbles. While only one gas compressor 51 is shown, obviously two or more stages of compression (not shown) could be added in series in place of compressor 51. I

In Figure 2, it will be seen that at a point intermediate the length of drill string 11, there is connected into the drill string an intermediate drill string section generally designated as 56. Intermediate drill string section 56 (Figure 3) comprises a drill pipe 57 having a conduit 58 therethrough with an inlet 59 and an outlet 61 adapted to be connected to other drill pipes 62 and 63 respectively.

As shown best in Figure 3, there is a branch conduit 64 connecting the interior 58 of said drill pipe with the exterior thereof, and there is a valve generally designated as 66 in said branch conduit controlling flow therethrougn Means comprising an expansible fluid motor comprising a bellows 67 is provided with means comprising conduit 68 and valve 69 for filling and sealing a compressed fluid inside said bellows 67 to provide a predetermined pressure therein at the temperature to be expected in operation in the well. Valve 69 can be operated through coupling 59 before section 56 is connected in the drilling string, however, if desired, valve 69 can be designed to be operated through opening 71, for example, in the same manner as the valve of an automobile tire, and if desired, a suitable plug (not shown) can also be provided to close opening 71 after bellows 67 .has been charged with compressedv fluid.

While valve 66 may be a simple valve with a single seat located in a simple by-pass passage, it is preferred to use the double-balanced, valve shown, and in order to use this balanced valve, branch conduit 64 is bifurcatedso that it has a first passage 72 connected by two other passages 73 and 74 to the opposite side of the wall of said drill pipe 57, and said valve 66 comprises a seat 76 and 77 in each of said passages 73 and 74 respectively, and a valve stem 78 actuated by said motor, said valve stem carrying valve heads 79 and 81 to close each of said seats 76 and 77 respectively at the same time, whereby pressure across said valve seats is balanced out. i

It will benoted that passage 73 serves another function of transmitting the pressure of the hydrostatic head of liquid in the well bore adjacent said bellows 67 to the exterior thereof so that it will be balanced against the pressure fluid contained inside the bellows 67.

As pointed out above, it is preferred to operate with the circulation in the direction shown in Figure 1, chiefly because this insures that the pressure in the well bore 82 (FigureZ) is less than the pressure in drill string 11 in chamber 58, which is an advantage, as the entire invention is devoted to lightening the hydrostatic pressure in the well bore. However, as pointed out above, it is possible to operate with reverse circulation in the 0pposite direction to the arrows shown in Figure 2 in which case the gas lightened liquid drilling fluid would enter the system through pipe 12, and proceed to the bottom of the bore 82, enter the fluid passage 46 in hit 47and return to the mud tank 14 through drill string 11. could be done with the apparatus as shown in Figure 3, as valve 79 and 81 would stay open allowing some of the drillingfluid to enter conduits 73 and 74 and pass through 72 into space 58 until such time as the hydrostatic pressure transmitted through opening 73 was insuflicient to prevent the compressed gas in bellows 67 from expanding and closing valve 79 and 81. When the valve 79 and 81 closes the 'drilling mud would have to allproceed down well bore 82 into holes 46 and up the interior of drill string 11.

It is preferred, however, to have the circulation in the direction indicated by the arrows. The liquid drilling mud from tank 14 is pumped by pump 18, or in the modificationashown in Figure 1 by pumps 18 and 54, through pipe 1 3, and is lightened by gas 52 compressed by compressor 51 and injected at 53. Well bore 82 containing liquid has a sufficient hydrostatic head adjacent bellows 67 to keep the same compressed and valves 79 and 81 open. As the gas lightened liquid drilling fluid passes down drill string 11, some, or all, of it emerges through passages 72, 73 and 74 to the well bore 82 at a point where the hydrostatic head ismuch less than down at bit 47 and holes 46. The drilling fluid returns to the surface through well bore 82 and pipe 12 and the gas therein rapidly lightens the fluid column in the upper portion 83 of the well bore above 56 which decreases the hydrostatic head adjacent 56and also decreases the hydrostatic head adjacent bit 47. When the hydrostatic head adjacent bellows 67 is lowered sufliciently, the fluid inside the bellows expands and moves valve stem 78 to close valves 79 and 81. When these valves close the gas lightened drilling liquid then proceeds inside drill string 11 all the way down to bit 47 and emerges from the holes 46 and thereupon lightens the hydrostatic head of the lower portion 82 of the well bore as well'as the hydrostatic head in the upper portion 83 of the same.

It will be seen that this accomplishes all the objects of the invention, by reducing the size and pressure of the gas compressor necessary to pump gas out holes 46 in the bit, by reducing the hydrostatic head ofv liquid in the well bore at the bit, by reducing the loss of drilling fluid to the formation caused by excessive hydrostatic pressure, and by reducing the loss of liquid caused by allowing the gas to produce a violent discharge of liquid at the surface of the ground before the flow becomes stabilized. It is obvious that all the other objects and advantages which have been pointed out, and which are apparent to those skilled in the art, will also be accomplished.

It is obvious that changes may be made in the illustrative embodiments shown in the drawing without de parting from the present invention. For example, in Figure 3, conduit 72 could discharge to the exterior 82 of drill pipe 57 and conduit 73 and 74 could discharge to the interior of 58 of said drill pipe, in which case bellows 67 when valves 79 and 81 were closed would be responsive to the adjacent pressure in chamber 58 rather than the adjacent pressure in well bore 82. However, the most preferredembodiment of the invention has beenshown in the drawing and described in the specification for illustrative purposes.

If desired, two or more by-pass valves 56 may be employed inserted in series at spaced intervals in drill string 11 in' the manner shown in Figure 4 with lengths of drill string 11 between them, to further distribute the gas through the column of mud in the well bore 82, 83 to prevent surges thereof, and for all the other objects and advantages set forth above with relation to the use of a single valve 56. The pressure in bellows 67 of each of the plurality. of valves 56 is adjusted by placing gas This in the same at about the same pressure as the hydrostatic head should be'in the welllbore adjacent each respective valve when the respective valve closes and the gas goes all the way down drill string 11 and out openings 46 in hit 47 in the final stage of operation of the device.

The plurality of'valves 56 are preferably spaced at equal intervals in the drill string 11 when assembled and in drilling position, as the hydrostatic head of the mud in well bore 82, 83 is about proportional to the dep of the respective valve 56. 1

Having described my invention, I claim:

l. The process of establishing circulation of a gas lightened drilling fluid through a drill string and bit and a liquid containing well bore containing the same, comprising the steps of circulating liquid through the drill string and well bore, injecting compressed gas into said liquid to lighten the same before it enters the drill string, establishing a shortened path of circulation for the re- 2. The process of reducing the loss of liquid drilling fluid to a formation in a well bore being drilled by the rotary well drilling process with a drill string and drill bit due to an excessive hydrostatic head of liquid in the well bore above said formation comprising the steps of circulating liquid through the drill string and well bore,

injecting compressed gas into said liquid to lighten the.

same before it enters the drill string, establishing a shortened path of circulation for the resulting gas lightened liquid between the interior of the drill string and the well bore at a point intermediate the length of the drill string, continuing the circulation of gas lightened liquid through said shortened path until the pressure of the hydrostatic head of liquid adjacent said point is less than a predetermined pressure, closing said shortened path and establishing circulation of said gas lightened liquid through said drill string, said bit, and said well bore, and con-' tinuing the rotary drilling operation while circulating said gas lightened liquid through said drill string, said bit, and said Well bore, whereby the reduction of hydrostatic head at said formation is suflicient to reduce the loss of liquid thereto.

3. An intermediate drill string section comprising a first drill pipe having a conduit therethrough with an inlet adapted to be connected to another drill pipe, an outlet adapted to be connected to still another drill pipe, and a branch conduit connecting the interior of said first drill pipe with the exterior thereof, a valve in said branch conduit controlling flow therethrough, and an expansible motor adjacent said valve for containing a fluid under a predetermined pressure the relatively expansible portions of said motor being connected to said first drill pipe and I to said valve to close said valve when the pressure exterior to but adjacent said motor is less than said termined pressure.

4. The combination of claim 3 in which said branch conduit is bifurcated, having a first passage connected by two passages to the opposite side of the wall of said drill pipe, and said valve comprises a seat in each of said two passages and a valve stem actuated by said motor, said valve stem carrying a valve head to close each of said seats at the same time whereby pressure across said valve seats is balanced out.

5. The combination of claim 3 in which said motor comprises a bellows with means for filling and sealing a predecompressed fluid inside said bellows to provide. said pre- I determined pressure.

6. A rotary well drilling system comprising in combination a rotary well drilling rig including a first drilling liquid pump having a first intake conduit and a first discharge conduit, and a well drilling string connected to said first discharge conduit to receive liquid drilling fluid circulated by said first liquid pump, a source of gas, a gas compressor having a second intake conduit connected to said source and a second discharge conduit connected to said first discharge condiut whereby compressed gas is combined with said liquid drilling fluid, a second liquid pump in said first discharge conduit downstream of said second discharge conduit to further compress said gas and-furtherraise the pressure on said drilling fluid, a bypass conduit in the wall of said drilling stiring, said bypass conduit connecting the interior and exterior of said drilling string adjacent a point intermediate the length of said drilling string, a valve in said by-pass conduit controlling-flow therethrough, and means comprising a motor for containing a fluid under a predetermined pressure connected to said drilling string and to said valve adjacent said valve to close said valve when the pressure exterior to but adjacent said motor is less than said predetermined pressure.

7. The combination of claim 6 in which said by-pass conduit is bifurcated, having a first passage connected by two passages to the opposite side of the wall of said drilling string, and said valve comprises a seat in each of said two passages and a valve stem actuated by said motor, said valve stem carrying a valve head to close each of said seats at the same time whereby pressure across said valve seats is balanced out.

8. A rotary well drilling system comprising in combination a rotary well drilling rig including a drilling liquid pump having a first intake conduit and a first discharge conduit, and a well drilling string connected to said first discharge conduit to receive liquid drilling fluid circulated by said liquid pump, a source of gas, a gas compressor having a second intake conduit connected to said source and a second discharge conduit connected to said first discharge conduit whereby compressed gas is combined with said liquid drilling fluid, a by-pass conduit in the wall of said drilling String, said by-pass conduit connecting the interior and exterior of said drilling string adjacent a point intermediate the length of said drilling string, a valve in said bypass conduit controlling flow therethrough, and means comprising a motor for containing a fluid under a predetermined pressure connected to said drilling string and to said valve adjacent said valve to close said valve when the pressure exterior to but adjacent said motor is less than said predetermined pressure. V

9. A rotary well drilling system comprising in combination a rotary well drilling rig including a first drilling liquid pump having a first intake conduit and a first discharge conduit, and a well drilling string connected to said first discharge conduit to receive liquid drilling fluid circulated by said first liquid pump, a source of gas, a gas compressor having a second intake conduit connected to said source and a second discharge conduit connected to said first discharge conduit whereby compressed gas is combined with said liquid drilling fluid, a second liquid pump in said first discharge conduit downstream of said second discharge conduit to further compress said. gas and further raise the pressure on said drilling fluid, a bypass conduit in the wall of said drilling string, said bypass conduit connecting the interior and exterior of said drilling string adjacent a point intermediate the length of said drilling string, a valve in said by-pass conduit controlling flow therethrough, and means connected to said drilling string and to said valve adjacent said valve to close said valve when the pressure exterior to but adjacent said means is less than said predetermined pressure.

10. A rotary well drilling system comprising in combination a rotary well drilling rig including a drilling liquid pump having a first intake conduit and a first dis charge conduit, and a well drilling string connected to said first discharge conduit to receive liquid drilling fluid circulated by said liquid pump, a source of gas, a gas compressor having a second intake conduit connected to said source and a second discharge conduit connected to said first discharge conduit whereby compressed gas is combined with said liquid drilling fluid, a by-pass condit in the wall of said drilling string, said by-pass conduit connecting the interior and exterior of said drilling string adjacent a point intermediate the length of said drilling string, a valve in said by-pass conduit controlling flow therethrough, and means connected to said drilling string and to said valve adjacent said valve to close said valve when the pressure exterior to but adjacent said means is less than said predetermined pressure.

11. The combination of claim 3 in which there is at least a second branch conduit connecting the interior of one of the other drill pipes with the exterior thereof at a point at a diiferent elevation and spaced from said first branch conduit, a second valve in said second branch conduit controlling flow therethrough, and a second expansible motor for containing a second fluid under a second predetermined pressure connected to said second valve and the drill pipe supporting the same adjacent said second valve to close said second valve when the pressure exterior to but adjacent said second motor is less than a second predetermined pressure.

12. The combination of claim 10 in which there is a second by-pass conduit in the wall of said drilling string connecting the interior and exterior of said drilling string adjacent a second point at a diflerent elevation and spaced from the first mentioned by-pass conduit, a second valve in said second by-pass conduit controlling flow therethrough, and means connected to said drilling string and to said second valve adjacent thereto to close said second valve when the pressure exterior to but adjacent said means is less than said predetermined pressure.

13. The process according to claim 1 wherein said liquid drilling fluid comprises an aqueous liquid mixture containing sutficient solids to form a filter cake on the wall of said well bore, and wherein the volume of said injected compressed gas is sufficient to lighten said fluid to a predetermined weight per unit volume.

14. The process according to claim 1 wherein said liquid drilling fluid comprises a water base drilling mud containing sufficient clayey material to form a filter cake on the wall of said well bore, and wherein the volume of said injected compressed gas is suflicient to lighten said fluid to a predetermined weight per unit volume.

15. The process according to claim 1 wherein said gas is an.

16. In the art of drilling a well bore in a formation, comprising the steps of drilling said formation with well drilling tools comprising a drill string and drill bit, circulating a liquid drilling fluid through said well bore and drill string and drill bit, and injecting compressed gas into said drilling fluid to lighten the same before it enters one of said drill strings and well bore, the improvement comprising the steps of circulating the resulting gas lightened drilling fluid by establishing a shortened path of circulation between the interior of said drill string and said well bore at a point intermediate the length of said drill string, continuing the circulation of said gas light ened drill fluid through said shortened path until the pressure of the hydrostatic head adjacent said point is less than a predetermined pressure, closing said shortened path when said predetermined pressure is attained and establishing circulation of said gas lightened drilling fluid through said drill string, drill bit and well bore, and continuing the drilling of said formation while circulating said gas lightened drilling fluid through said drill string, drill bit and well bore, whereby the reduction of hydrostatic head in said well bore results in a substantial increase in the rate ofdrilling said formation.

17. In the art of drilling a well bore in a formation, comprising the steps of drilling said formation with well drilling tools comprising a drill string and drill bit, circulating a liquid drilling fluid through said well bore and drill string and drill bit, and injecting compressed gas into said drilling fluid to lighten the same before it enters said drill string, the improvement comprising the steps of initially circulating the resulting gas lightened drilling fluid through a first shortened path between the interior of said drill string and well bore at a point intermediate the length of said drill string, continuing the circulation of said gas lightened drilling fluid through said first path until the pressure of the hydrostatic head adjacent said point is less than a predetermined pressure, closing said first path when said predetermined pressure is attained, progressively circulating said gas lightened drilling fluid through a plurality of second shortened paths located a progressively greater depths in said drill string intermediate the length thereof, progressively closing each of said second paths when the pressure of the hydrostatic head adjacent each of said second paths is less than a predetermined pressure before establishing circulation of said gas lightened drilling fluid through the next of said 10 second paths located at a progressively greater depth, finally establishing circulation of said gas lightened drilling fluid through said drill string, drill bit, and well bore after the last of said second paths is closed, and continuing the drilling of said formation while circulating said gas lightened drilling fluid through said drill string, drill bit, and well bore, whereby the reduction of hydrostatic head in said Well bore results in a substantial increase in the rate of drilling said formation.

18. The process according to claim 1 wherein said liquid drilling fluid comprises an oil base drilling mud containing sufficient solids to form a filter cake on the Wall of said well bore, and wherein the volume of said injected compressed gas is sufficient to lighten said fluid to a predetermined weight per unit volume.

References Cited in the file of this patent UNITED STATES PATENTS 2,082,329 Foran et a1. June 1, 1937 2,179,226 Bryan Nov. 7, 1939 2,465,060 Carlisle et al Mar. 27, 1949 2,537,605 Sewell Jan. 9, 1951 2,555,265 Bergman May 29, 1951 2,726,063 Ragland et al Dec. 6, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,833,517 May 6, 1958 Roy A, Bobo It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 56, for "drill" read drilling column 6, line 13, after "lightened" insert liquid column 9, line 20, for "a" read at Signed and sealed this 24th day of February 1959,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Oflicer 

1. THE PROCESS OF ESTABLISHING CIRCULATION OF A GAS LIGHTENED FLUID THROUGH A DRILL STRING AND BIT AND A LIQUID CONTAINING WELL BORE CONTAINING THE SAME, COMPRISING THE STEPS OF CIRCULATING LIQUID THROUGH THE DRILL STRING AND WELL BORE, INJECTING COMPRESSED GAS INTO SAID LIQUID TO LIGHTEN THE SAME BEFORE IT ENTERS THE DRILL STRING, ESTABLISHING A SHORTENED PATH OF CIRCULATION FOR THE RESULTING GAS LIGHTENED LIQUID BETWEEN THE INTERIOR OF THE DRILL STRING AND THE WELL BORE AT A POINT INTERMEDIATE THE LENGTH OF THE DRILL STRING, CONTINUING THE CIRCULATION OF GAS LIGHTENED LIQUID THROUGH SAID SHORTENED PATH UNTIL 